Here's an idea .take your stock kit windows .Cut the runners off between the front and back if they have them .Then cut the vent windows off the windshield part . Now , take some Florist'sclay and build a box of it behind the part .

Make sure the edges are not undercut but vertical but slightly wider at the bottom . Now take .010sheet and lay a piece on there .gently pass a hair dryer on high over it until it conforms to the window .Spray with cool water and remove .Trim carefully and there you have an optically clear and in scale windshield or back window.

I developed this method because I don't have room for a large Vac-Form machine and there were many of Johan's kits that had,like tha actual cars the excessively bubbled wondows ( read Chrysler here )

I worked in vacuum forming nearly 40 years. There are simple designs for simple ones to build all over the internet.

I built several for home use. Nothing fancy - they don't have to be.

The one I liked best was using heater elements from a toaster.

Like all home made things you have to experiment a little.

If you don't like messing woth electricity use your home oven to heat your sheet of plastic. Works good for other things besides windshields. You can actually form entire bodies. Again, the internet helps tons.